1. Field of the Invention
The invention relates to a method of controlling the rotational speed of the drum of program controlled laundry treatment machines such as, for instance, washing machines, washer-dryers and dryers, of the kind provided with a drum rotatable about an at least approximately horizontal axis, a drive motor for the drum, a measuring device for defining the load parameters as a function of the laundry put into the drum, and with a control device for setting the drive motor for different revolutions per minute during the various cycles of a washing or drying program, such that during part of a washing or drying program individual rotating cycles are carried out with intermittent idle periods, the drum being driven during a rotational cycle at different rotational speeds ranging between upper and lower values. Unless otherwise indicated hereinafter, machines of the kind under consideration will be referred to as laundry machines.
2. The State of the Art
When textiles are being washed, the mechanical action affecting the laundry is one of the significant factors in terms the result of the washing operation. To provide the most efficient washing action in a washing machine equipped with a horizontally or at least approximately horizontally suspended rotary drum, individual pieces of the laundry should be moved to about the 12-o'clock-position and then, upon release from the wall of the drum, drop down in consequence of gravity. This is brought about by the centrifugal force of the laundry being slightly less than the gravity of the earth. Since the centrifugal force is a function of the distance of a piece of laundry from the rotational axis of the drum, a rotational cycle implemented at a predetermined constant rotational speed is of advantage only in respect of those laundry pieces which have moved to a certain distance from the axis of the drum. The standard value of this distance is assumed to be the same as the radius of the drum. Laundry closer to the axis of the drum drops substantially sooner, i.e. it separates at the 9- or 10-o'clock-position, and instead of a dropping movement, it goes through a rolling movement.
From DE 34 36 786 A1 it is known during washing to drive the drum at a speed dependent upon the quantity of laundry such that with increasing quantities of laundry, higher rotational speeds are applied. Thus, those items of the laundry which are at a large distance from the rotational axis are subjected to strong gravitational forces and they form a ring engaging the wall of the drum. This prevents the laundry from dropping freely.
From DE 39 33 355 it is known during a laundry operation to provide cycles of alternating rotational directions with intermittent idle periods, the drum being initially rotated at an upper speed of 55 min−1 followed by a lower speed of 40 min−1. The purpose of the higher speed is to subject the laundry to satisfactory mechanical action, and the lower speed is to ensure sufficient soaking of the laundry.
DE 100 05 991 A1 discloses a washing machine provided with a measuring sensor, such as a spring scale, for defining a load step corresponding to the weight of the laundry deposited into the drum.
A washing machine is known from DE 44 38 760 A1 in which, based on the oscillations of the signal of rotations during a reverse cycle, a measuring device defines a load step depending on the kind and quantity of the laundry placed into the drum.
In the washing machine W 487 WPS manufactured and sold by the assignee, the drum, during the “boiling-/colored laundry” cycle of a wash program, is operated in accordance with the rotational cycle known from DE 39 33 355 A1, and it is provided with the weight measuring feature in accordance with DE 100 05 991 A1. In the washing machine W 453 WPS manufactured and sold by the assignee, the drum, during the “boiling-/colored laundry” cycle of a wash program, is also rotated in accordance with the cycle known from DE 39 33 355 A1, but it is provided with the load step recognition known from DE 44 38 760 A1.
DE 100 14 718 A1 discloses a laundry dryer provided with a feature for detecting the pattern of laundry movement as a function of the laundry placed in the dryer. The dryer is equipped with controls for energizing the drive motor of the drum in response to the movement pattern of the laundry in the drum such that a desired pattern of laundry movement in the drum may be set by way of the rotational speed of the drum. Its purpose is during the drying process to move the laundry in the drum through the heated air in a predetermined trajectory. While suitable for laundry consisting of large sheets, as used in hospitals, for instance, controlling the rotational speed of the drive motor of the drum as a function of the pattern of laundry movement is unsuitable for household laundry.